The present invention relates generally to radiological well logging methods and apparatus for investigating subsurface earth formations traversed by a borehole and, more particularly, to improved method and apparatus for high frequency pulsed neutron gamma ray logging wherein gamma radiation resulting from neutron inelastic scatterings and thermal neutron capture are selectively detected and the spectral distribution of the detected gamma radiation is determined.
A number of well logging techniques in use come under a general classification of neutron induced radiological well logging. In their basic form, these techniques involve irradiating subsurface earth formations with neutrons and then determining the effect of select formation constituents on the neutrons by measuring either gamma rays produced by inelastically scattered neutrons or gamma rays resulting from thermal neutron capture. The detection of these radioactive signals returning to the borehole provides information as to porosity, lithology and presence or absence of hydrocarbons within the investigated formations.
In the neutron induced gamma ray well logging instrument commonly used, a neutron generator is pulsed at a rate up to approximately 20 KHz. Each neutron burst produces inelastically scattered gamma radiation which is detected during a time interval coinciding with each emission of neutrons from the neutron source and thermal neutron capture gamma radiation which is detected during a time interval subsequent to the cessation of each neutron burst. A detection pulse is transmitted to surface electronics for processing during each of the two detection intervals along with a synchronization pulse. The synchronization pulses are used in the surface electronics to route the pulse occurring during the inelastically scattered detection interval to one section of a multi-channel analyzer. Similarly, the pulse occurring in the thermal neutron capture detection interval is routed into another section of the multi-channel analyzer. The use of memory, address decoders, subtractors and ratio circuits enables information to be obtained indicative of selected borehole and formation characteristics.
It is known that at a pulsed source repetition rate of approximately 20 KHz, the neutron induced gamma ray well logging instrument must traverse the borehole at a rate of speed of less than five feet per minute to obtain reliable information. At increased logging speeds the information obtained is significantly inferior due to insufficient numbers of neutrons bombarding the formation intervals and decreased count rates of the detected gamma rays. Thus increase in logging speed requires a proportional increase in the pulse repetition rate of the neutron source.
Increasing the pulse repetition rate of the neutron source significantly decreases the time available for the detection of returning radiation. As a result the time interval separating the inelastic gamma ray detection interval and the thermal neutron capture gamma ray detection interval is greatly reduced. A reduction in the time interval separating the two detection intervals results in, the pulses detected within each interval being more likely to pile-up or spread into one another during transmission to the surface electronics over the logging cable. Such pulse spreading results in what appears at the surface to be one large pulse rather than separate and independent pulses. The energies measured in the distorted pulse are no longer directly related to elements within the neutron irradiated subsurface earth formations and thus are unsuited for use in evaluating these formations.
Accordingly, at increased pulse repetition rates of a neutron source it is difficult to establish accurate measurements of the borehole and formation characteristics. This and other disadvantages are overcome with the present invention by providing a method and apparatus for operating a pulsed neutron source at relatively high repetition rates without suffering pulse pile-up of the detected pulses during transmission over the logging cable.